An error message is seen on the Australian Bureau of Statistics census website on Wednesday.
Photograph: Joel Carrett/AAP

The federal government and Australian Bureau of Statistics (ABS) have explained the outage of the online census was the result of a systems failure and an “overcautious” response to a denial of service attack.

At a press conference on Wednesday to explain the outage since about 7.30pm on Tuesday, the small business minister, Michael McCormack, blamed the failure on a “confluence of events” but said the system had not been breached and no data was lost.

The actor behind the attack has not been identified and the ABS is still working to prepare the online system to be put back online.

At a separate press conference in Sydney the prime minister, Malcolm Turnbull, said the ABS site had not been hacked and announced a review of the events.

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“I want to assure Australians that the unequivocal advice we have received from IBM, from the ABS, from the Australian Signal Directorate, is that their Australian census data is safe, it has not been compromised,” he said.

Turnbull said the cybersecurity chief, Alastair MacGibbon, would head a “very thorough review of the events”, supported by the Australian Signal Directorate, treasury and ABS.

But Labor leader, Bill Shorten, and Nick Xenophon are not satisfied with the proposed review, and have both called for the Senate to review the bungled census.

And in comments creating trouble for his own government, conservative backbench senator, Cory Bernardi, described the census as a “debacle” caused by a system that “certainly wasn’t robust”.

According to McCormack, the ABS census site suffered a large scale denial of service attack at 7.30pm, which led to a hardware failure, the overload of a router and a false alarm from the system about the attack.

He said at 7.45pm the ABS then took a “very cautious approach to safeguard the data submitted” by taking the census system down.

That followed a number of denial of service attacks earlier in the day that had been defeated by the system provided to the ABS by IBM, including a decision to block international traffic access to the census form at 11.46am.

But MacGibbon said the geoblock was defeated by traffic from the United States from an unidentified actor.

MacGibbon said an investigation was needed to determine who was behind the denial of service. He said it was designed to “cause frustration”, which he conceded it had.

The small business minister, Michael McCormack, and the ABS chief statistician, David Kalisch, face the media on Wednesday to explain the failure of the census website on Tuesday night. Photograph: Adrian Muscat/AAP

The ABS chief statistician, David Kalisch, said there was “no issue with the capacity” of the census system. The ABS was still receiving 150 forms per second, fewer than the 260 forms a second it was built to handle, when it was taken down, he said.

But he said the critical denial of service attack “breached the online form because it didn’t get caught by geoblocking”.

Kalisch said the online form would be restored “as soon as we are assured it is robust and secure” and the ABS was taking advice from Australian Signal Directorate on the issue.

McCormack said 2.33m online forms were submitted before the outage and no data had been lost.

Kalisch apologised for the inconvenience but said the ABS had taken the “early, prudent precaution to take the system down to be assured of the integrity of the data”.

Asked why he had reassured Australians at 3pm on Tuesday that the census was proceeding as planned, Kalisch said that Australians would “not be surprised” that a government system was targeted by a denial of service.

He added that he “didn’t think it was appropriate” to disclose the denial of service attacks, which, to that point, had been successfully identified and managed.

McCormack sought to distance himself from the failure by noting he had only become small business minister, and gained responsibility for the census, three weeks ago.

Asked why he had got the job so shortly before the census, he said the census had been planned for five years, in comments that may be seen to shift responsibility to the former assistant treasurer Kelly O’Dwyer and the former assistant minister to the treasurer Alex Hawke.

The treasurer, Scott Morrison, said the ABS had an unblemished record when it came to security of census data and had taken every step on Tuesday to protect that record.

Opposition leader, Bill Shorten, called it “not only the worst-run census in the history of Australia, but without a doubt one of the greatest IT bungles and stuff-ups that a commonwealth government has ever been associated with”.

He said the Senate, not just the ABS, should inquire into the census issues.

The shadow assistant treasurer, Andrew Leigh, called for a “full transparent and open inquiry”, saying he was “not sure Australians can have faith in very much that the Turnbull government says about the census”.

“No less than 14 times over the last year I’ve warned about inadequate resourcing to the ABS,” he said. “Labor was deeply concerned with the position of chief statistician was left vacant for nearly a year.

In his weekly newsletter on Wednesday Bernardi said he wasn’t surprised the system crashed because “much bigger IT efforts have succumbed to the weight of server demands in both the government and commercial worlds”. He questioned “how our government agencies thought we would be any different”.

“I only wonder if anyone will actually be held accountable for the failings. Time will tell,” he said.

He ridiculed government claims the system had not been hacked or attacked as “a bit like Monty Python’s black knight saying they had only suffered a flesh wound”.

Xenophon also wants a Senate inquiry into the census when sittings resume in Canberra on 30 August, and warned the government against a “dismissive” approach.

Cabinet minister Christopher Pyne, who is responsible for the Australian Signal Directorate, said it was “just not right” to characterise the census as a failure and dismissed claims the intruders were Chinese.

“Let’s not have an orgy of anti-Chinese xenophobia because of claims being made of this or that,” Pyne told reporters in Cairns.

Australians have until well into September to complete their forms and won’t be fined for late lodgement.